1. Field of the Invention
The present invention relates to a method of producing spherical silica powder which can be used for such purposes as a filler in semiconductor encapsulants.
2. Prior Art
Silica powders are used as fillers in semiconductor encapsulants. The increasingly high level of integration in semiconductors has created a need for encapsulants with better mold-filling properties that now makes it essential to use spherical fused silica powders. A number of methods for producing spherical fused silica powder by introducing silica powder to a burner flame are described in the prior art. For example, JP-A2-199013 (allowed as JP 2670628) and JP-A 10-95607 disclose processes of passing a starting silica powder through a flame formed by the combustion of a flammable gas and oxygen, thereby melting and spheroidizing silica particles.
In general methods customarily used in the art, the starting silica powder is fed to a burner flame formed by the combustion of oxygen with a flammable gas such as propane. The combustion flame imparts heat to the powder, causing it to melt and spheroidize. Thermal power sufficient to raise the temperature to the melting point or higher is required to melt the silica powder. In addition, a long flame length is needed to uniformly melt silica powder having a broad particle size distribution. For these reasons, use is made of a flammable gas which generates more heat than the amount theoretically required to melt the starting silica.
However, using large amounts of flammable gas and oxygen lowers the thermal energy efficiency, in addition to which it raises costs and is thus uneconomical. On the other hand, holding down the thermal power of a combustion flame shortens the length of the flame. In a flame of this type, fine silica particles can be spheroidized because they have a low heat capacity and a long retention time in the flame. However, coarse silica particles do not readily spheroidize on account of their large heat capacity and their short retention time in the flame. Accordingly, a need has been felt for a cost-effective method of producing spherical fused silica powder of any particle size suitable for use as a filler in semiconductor encapsulants which requires less flammable gas and has a better thermal energy efficiency.
It is therefore an object of the invention to provide a method which is capable of cost-effectively producing spherical fused silica powder that can be used for such purposes as a filler in semiconductor encapsulants.
After thoroughly investigating methods for feeding silica powder to a flame and continuously melting and spheroidizing the powder in the flame, the inventor has found that using a flame formed by the oxidative combustion of a non-halogenated siloxane or alkoxysilane as an auxiliary flame in the production process boosts heat generation, reduces the amount of flammable gas required, and increases the flame temperature and length. The result is a higher thermal energy efficiency and excellent cost effectiveness. Moreover, the core particles of silica that form as a result of siloxane or alkoxysilane combustion coalesce and grow on surfaces of silica particles, promoting spheroidization.
Accordingly, the invention provides a method of producing spherical silica powder, in which method silica powder having an average particle size of 0.3 to 40 xcexcm is fed to a burner flame formed with a flammable gas so as to continuously melt and spheroidize the particles within the flame. A non-halogenated siloxane and/or a non-halogenated alkoxysilane is fed to the flame as an auxiliary flame-generating source.